Neisseria meningitidis is serologically classified into serogroups, serotypes and immunotypes based on capsular polysaccharides, outer-membrane proteins and lipooligosaccharides (LOSs), respectively. The LOS is one of the major surface antigens and can be divided into 12 immunotypes. Epidemiological studies have revealed that the L3 immunotype is the most common immunotype among serogroups B and C disease strains. We have been investigating the potential use of nontoxic LOS-based protein conjugates as vaccines for the prevention of group B disease, which currently has no licensed vaccines. We selected an OP- mutant LOS instead of the L3 LOS to synthesize the LOS-based protein conjugate for two reasons: (1) Rabbit antisera to the OP- LOS cross-react with the L3 LOS and (2) unlike the L3 LOS, the OP- LOS is devoid of the lacto-N-neotetraose (LNnT), a precusor of blood group antigens. Thus, there is no risk of inducing antibodies by the conjugate that may cross react with the LNnT-containing glycolipids (such as paragloboside) on human tissues. We selected deoxycholate-treated outer-membrane protein (DOMP)from OP- strain as the carrier protein for the synthesis of the conjugate. None toxic oligosaccharide (OS) was conjugated to DOMP based on last year's scheme. The resulting conjugate was immunogenic in mice and rabbits. IgG antibodies to the native LOS were induced in both animals. During the study of the conjugate vaccine, we found that uncojugated OP- DOMP alone also induced good antibody titers. This OP- DOMP contained 10-20% tightly bond LOS. Since it is much easier to prepare D-OMP than synthesizing the OS-DOMP conjugate and the toxicity of LOS in DOMP is highly attenuated, we are currently examining the immunogenicity of OP- DOMP in rabbits and in mice. In this project, we have also developed a new quantitatve chemical assay for LPS. The new method can quantify LPSs of Bordetella pertussis, H. infuenzae and Vibrio cholerae; which did not a reliable colorimetric chemical method prior to this new developed assay.